There are many things that can cause the symptoms you mention. I would work through the basics and then go to more advanced items.

Maybe some of the pictorials will help you.

http://dieselgiant.com/mercedes_diesel_maintenance_tips.htm

Ask if you dont understand anything.

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1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair